The present invention relates to valves in industrial processes, and more particularly, to detection and diagnosis of fluid leaks through closed valves.
In the process control industry, automated control valves are used extensively to control process fluid mass flow and/or velocity in industrial processes. In some instances, especially in batch processes, it is necessary that a valve achieve a tight shut-off condition when it is closed. The phrase “tight shut-off” refers to a valve position wherein zero or near-zero fluid flows through the valve. In particular, a tight shut-off condition exists where no fluid flows through the valve, or where fluid flow is reduced to such a level that the flowing fluid had negligible impact the process.
In industrial process where a tight valve shut-off condition is required, if the valve does not shut-off tightly, the resulting material leakage into a batch recipe can ruin the batch. If a tight shut-off valve is leaking a noxious or toxic chemical, the leak can present a hazard for plant personnel and may result in an incident requiring involvement of the Environmental Protection Agency (EPA). Both of these outcomes can be very expensive.
Tight shut-off of a control valve is usually achieved using seals, such as elastomeric seals or Teflon® seals. For corrosive process fluids, Teflon® and other corrosion resistive materials are preferably used as the seal material. Unfortunately, seals fail for a variety of reasons, including corrosion, fouling, cavitation, physical wear and the like. Corrosion typically erodes the seal creating surface imperfections that make a tight seal difficult to achieve. Fouling refers to a material build up on the surface of the valve seat or seal, which prevents the valve from achieving a tight shut-off. Cavitation refers to a localized formation within a fluid flow of air or vapor pockets that expand explosively within the valve due to lowering of pressure within the flow (such as when the valve is adjusted from a closed to an open position). Expansion of vapor pockets within the flow can cause metal erosion and eventual valve failure. “Physical wear” refers to an instance where a seal is damaged during the valve closing process by pinching material between the valve plug and the valve seat or seal, thereby damaging the seat or seal body. Finally, debris can also interfere with the seal or valve travel in general, thereby preventing tight valve shut-off.
There is an on-going need in the process control industry for a means of detecting when a valve seal or valve positioner has failed or if a tight shut-off valve is leaking. Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.